1. Field of the Invention
The invention relates generally to the field of optical surface inspection, and more particularly, to illumination and light collection optics for inspecting semiconductor wafers, mask substrates, and other similar articles.
2. Background Information
Monitoring anomalies, such as pattern defects and particulate contamination, during the manufacture of unpatterned silicon wafers is an important factor in increasing production yields. Numerous types of defects and contamination, especially particles, can occur on a wafer's surface. Determining the presence, location and type of an anomaly on the wafer surface can aid in both locating process steps at which the anomaly occurred and determining whether a wafer should be discarded.
Originally, particles were monitored manually by visual inspection of wafer surfaces for the presence of particulate matter. These particles, usually dust or other microscopic particles, caused many of the wafer pattern defects. However, manual inspection proved time-consuming and unreliable due to operator errors or an operator's inability to observe certain defects.
To decrease the time required to inspect wafer surfaces, many automatic inspection systems were introduced. A substantial majority of these automatic inspection systems detect particles based on the scattering of radiation. These systems include two major components: illumination optics and collection-detection optics. Illumination optics generally consists of scanning a wafer surface with a coherent source of radiation, e.g., a laser. Particles present on the wafer's surface scatter incident radiation. The collection optics detect the scattered radiation with reference to the known beam position. The scattered radiation is then converted to electrical signals which can be measured, counted and displayed as bright spots on an oscilloscope or other monitor.
The sensitivity of systems that employ radiation to detect particles is based on a ratio of the photon flux detected by radiation striking a particle to the photon flux detected by radiation striking the surface of the wafer. The greater this ratio, the greater the sensitivity of the system. Therefore, many systems utilize radiation that is incident on the wafer's surface at an oblique angle. Obliquely incident radiation generates a larger ratio of photon flux from a particle to that from the surface of the wafer. These systems are then able to better detect smaller particles on a wafer surface.
Known sample inspection systems can detect particles as small as sixty nanometers in diameter while still gathering other data allowing them to differentiate between particles and crystal originated particles (also known as “COPs”, which are surface breaking defects in a semiconductor wafer that were in the past classified as “particles” due to the inability of earlier inspection systems to distinguish them from real particles). An exemplary system of this type is the Surfscan® SP1TB1 Wafer Inspection System by KLA-Tencor, Inc. of San Jose, Calif. The Surfscan® system uses two separate radiation beams to inspect a substrate, one radiation beam that is substantially normal to the sample surface, and one that is at an oblique angle in the range of 70 degrees from normal to the sample surface. The oblique radiation beam typically has a 488 nm wavelength. Although systems such as this one are available, it is desirable to provide sample inspection tools that can detect even smaller particles and COPs without compromising their ability to differentiate between particles and COPs. It is further desirable to provide inspection tools to detect defects that are strictly on the surface of semiconductor wafers and other samples, and to distinguish these from those defects that are within the samples.